The polycladida are flatworms and are a marine dwelling animal. They are one of the few animals in their phylum that are not a considered to be a parasite. They are similar looking to a carpet and move much more in the dark than they do in the light. They sometimes can be seen on shore and are about one centimeter long but they can grow to be up to ten centimeters long. They breathe through pores that they have on their skin. The air comes through pours and is then filtered from there and enters the blood stream where it oxygenates the blood.